For ecological reasons, electric traction is a must for public transport in urban districts.
Overhead electricity supply lines entail putting up supporting structures and having costly and unsightly cable connections.
Besides their not very aesthetic appearance and their size, which clutter up the field of view and spoil the beauty of urban areas and monuments, catenaries present a risk of electrocution and form a height obstacle.
On the other hand public transport vehicles are being developed, which are energy-autonomous over more or less long distances. These are vehicles whose traction or propulsion is powered by electricity from electrochemical elements (batteries, supercondensers) but also by stored energy, for example in the form of kinetic energy supplied by a flywheel rotating at high speed, and other means.
Although such systems provide an interesting solution from an ecological standpoint and the present various nuisances, such as spoiling the urban landscape, their main drawback concerns their low energy storage capacity and their very moderate overall energy efficiency.
Besides, the cost of the equipment is augmented by the cost of energy accumulation means.
There are already autonomous systems which are recharged with energy at stop stations along their route and at terminal stations. These entail fairly long periods of vehicle immobility.
At these stops, energy for recharging is tapped from one or more ground rails.
For operational reasons the stop times at public transport stations cannot exceed the period of immobility that allows passengers to get off and on the vehicle.
It is desirable for that period to be as short as possible, so as not to lengthen the travel time. In addition, it is sought to reduce the period so as to improve the transit time still further.
Thus, since the recharging periods at each station are rather short, they can do little to improve autonomy.